Technical Abstract:
Administration of dexamethasone (DEX) to cattle is commonly used in models of stress-induced effects on the host defense, including those employed to investigate interactions of microorganisms with their host. Much less is known about the effects of DEX on the adaptive immune response in cattle than in other species. Previous studies showed that different bovine lymphocyte subsets are differentially affected by DEX. The objective of the present study was to characterize subsets of circulating lymphocytes in calves prior to and 48 h after the onset of parenteral DEX treatment. Treatment significantly reduced the overall percentage of circulating lymphocytes, and disproportionately depleted the population of gamma delta TCR**+/CD8alpha**- cells. Analysis within the CD8alpha**+ population of T cells further revealed that DEX treatment also reduced the CD8alpha**low subset of gamma delta T cells coexpressing the activation marker ACT-2**+. By contrast, DEX treatment did not affect the percentage of CD8alpha**low/CD25**+ cells, indicating that cells with a special activation state were affected. Although there was a sharp increase in the number of CD25**+ PBMC from DEX-treated calves, only marginal differences were noted in the percentages and the proliferative capacity of the major lymphocyte subsets. Despite a similar reduction of mRNAs for several Th-prototype cytokines (IL-2, IFN-gamma, IL-4, TGF-beta) in short term-PBMC cultures from DEX-treated calves, the amount of il-10 transcripts was not affected. The results of this study extend the evidence that DEX treatment does not generally suppress the bovine immune system but has a number of different effects on different lymphocyte subpopulations. This information must be considered when utilizing DEX treatment to improve bovine infection models.